Have a look on Auto Trader and see where the Age/Spec and Mileage fit in as well.
Are you specifically looking for a car that is already modified?
Opinions are probably divided on here about whether buying a modified car is the best idea, although it looks to have been owned by an enthusiast and 300 BHP for stage 2 does seems optimistic...typical figures are around 260 BHP.
As with any second hand sale there are lots of unknowns - If you paid more at a dealer and got a warranty you could have the car scanned for faults and would have some security if faults occur, in a private sale its sold as seen.
Is this your first MK5?